The present invention relates to an improvement in heat transfer technology whereas heat can be assimilated or dissipated from granules or other solid particles by convection at a remarkably rapid rate, in an extremely energy efficient manner, and with a reduction in pollution potential by particulate emissions, while maintaining a safer and more secure environment for those involved with the operation. Specifically the invention relates to heating, cooling, or drying granular or solid particles, as apart from liquid particles, while in a state of fall in a partially or totally enclosed container such that the rate of heat transfer to or from the granules or solid particles is high and the efficiency of the energy used in effecting the heat transfer is enhanced by forced recirculation of gas through the mass of falling solid particles by the use of gas movers mounted inside the container to pass gas through the mass of falling particles. More specifically, the present invention relates to an improvement in melt granulation processes wherein the heat released in crystallization of the melt and associated heat from cooling of the melt and subsequently solid material must be dissipated in some manner. Most specifically, it is a significant improvement of the invention disclosed earlier by Shirley in U.S. Pat. No. 4,213,924, issued July 22, 1980. In most cases the technique can also be applied to encapsulation and coating processes. Further, the present invention can be used in conjunction with existing technology to obtain greatly improved heating, cooling, and drying efficiencies and capacities from equipment whose design has been modified to use the invention which is disclosed herein. The process, procedures, and equipment of the invention apply to granulation of hygroscopic materials as well as those which are nonhygroscopic and to the heating, cooling, and drying of almost all solid particles.
In particular, the present invention relates to heating or cooling of solid particles in an enclosed vessel, such as a rotating drum wherein gas can be ventilated through said vessel and wherein by means of this invention, as the gas passes through the vessel, the gas can be forced to repeatedly contact the solid particles being heated, cooled, or dried. In specific cases as described in the examples presented later, air is used and it can be cooled by evaporation of water between forced contacts with the solid particles to increase the heat transfer rate and capacity of a rotary drum granulator. Those acquainted with the art of heat transfer can readily see that this type of technique also can be used to improve the heat transfer rate and capacity of a rotary drum heater or dryer by heating the air between forced contacts with the solid particles. The technique works best in a rotary drum in which lifting flights are present but which, when in operation, does not shower granules throughout the cross section as disclosed by Thompson et al in U.S. Pat. No. 3,398,191, issued Aug. 20, 1968, but rather is similar to Blouin's disclosure in U.S. Pat. No. 3,877,415, issued Apr. 15, 1975, in which a deflector pan or pans gather showering curtains of solid particles together in a continuous falling curtain. The earlier Shirley disclosure U.S. Pat. No. 4,213,924, supra, shows how such a rotary drum design can be used to create an area in the cross section of the rotary drum extending throughout the length of said rotary drum where water mist can be sprayed for evaporative cooling of air. A three dimensional space in a rotary drum or an enclosed or semi-enclosed container for purposes of clarity shall be henceforth defined herein as a section of said rotary drum or container. Those knowledgeable in the art of heat transfer can readily see that the same section which in one case might be used to cool air or some other gas could be used to heat same by direct contact with a heat source should it be advantageous to constantly or intermittently supply heat to the ventilating gas for any reason as it passes through the rotary drum, such as might be the case in a co-current or counter-current dryer.
Those acquainted with the art of drying and heating will quickly appreciate the advantages this invention offers in heating or drying heat sensitive solid particles at improved heat transfer rates with efficiency and without risk of overheating of the material.
Those skilled in the art are well aware of heat transfer technology as it applies to fluid beds and spouted-fluid beds such as have been disclosed by Niks et al in U.S. Pat. No. 4,219,589, issued Aug. 26, 1980, and by Kono et al in U.S. Pat. No. 4,217,127, issued Aug. 12, 1980, respectively. Fluid-bed technology is recognized to be one of the best heat transfer means between a gas and solid particles. The heat transfer rates within the bed are exceptionally high. A primary object of the instant invention is to emulate this extremely effective means of heat transfer by convection between gas and solid particles while overcoming several inherent problems of heat transfer in fluid-bed units. In true fluid-bed units, energy usage is quite high because of pressure drops across the gas distribution plate, bed of fluidized solid particles, pollution abatement equipment, and associated ductwork; and heat transfer efficiency, although quite good by some standards, is extremely low when measured by those of the instant invention. Fluid-bed units do not lend themselves to reuse of the air blown through a bed, although it is done in several multibed designs, but usually at great expense in fluidizing energy. It is also possible to pass exit gas through cooling and/or heating devices and return said gas through additional portions of the same fluid bed but usually this is associated with expensive cyclone and/or bag collectors to prevent solid particles from plugging the gas distribution screens.
The prior art also teaches the use of closed convection systems such as ovens where gas is blown lightly through porous beds of solid particles sometimes supported on traveling grates or screens. The gas can be reheated or cooled, and then recycled through the solids. This overcomes some of the fluid-bed problems in that it normally reduces gas usage and sometimes pressure drops involved in blowing the gas, but it introduces other problems such as much poorer gas and solid particle contact due to channelizing, reduced exposed surface of the solid particles, and more uneven cooling and heating.
In addition to the fluidized bed and the closed oven methods, supra, a third classical means of effecting heat transfer between gas and solid particles is the rotary drum or kiln. The rotary drum serves as an elevator lifting granules up by using flights and letting the solid particles fall back to the bottom of the unit through gas blown from one end of the rotary drum to the other. Kiln usually do not have lifting flights and therefore have even poorer heat transfer characteristics. Many efforts have been made to improve rotary drum heat transfer, but heat transfer rates have remained low, and the rotary drum units have remained inefficient and energy intensive.
The present invention amalgamates most of the best properties or characteristics of these systems into one superior means of effecting heat transfer between gas and solid particles. It is felt that this has been accomplished through the instant innovative approach. From the fluid bed was borrowed the principle that gas blowing through suspended solid particles in a more or less dense phase, as in a fluid bed, is the best means of contact for heat transfer purposes and not as gas contact occurs in long rotary drums where gas flow is axial sometimes passing through but mostly flowing parallel to the showers of falling solid particles. The present invention involves using falling solid particles usually formed into a multiplicity of curtains which approaches the dense phase characteristics of a fluid bed, but not characteristic of a typical rotary drum. Gas is passed through the curtains at angles greater than 45.degree. with the fall line of the solid particles to effect better mixing of gas and granules than can be effected in a typical rotary drum. Because fluidization of solid particles by gas is energy intensive whereas mechanical lifting can be much more economical, the use of a rotary drum for test work was chosen. It was recognized that other mechanical lifting devices such as a cleted conveyor, bucket, or flighted elevator, or any number of similar transport devices which can be part of an enclosed or semi-enclosed system, such as a rotary drum, or can be put into a partially or totally enclosed container may be used to effect heat transfer by the instant disclosure. Solid particles lifted vertically to be released to shower through the drum of container unimpeded or fashioned by baffles and/or screens or other devices into various patterns of falling material all are within the spirit, if not the content, of this disclosure. In some cases, solid particles may be lifted by mechanical means but not released to make a true fall. The conveying device will lower them in such a manner that the solid particles are essentially loose from each other and have an excellent position for effective convective heat transfer, but retained by restraining screens, open weave fabric, or slotted plates to prevent scattering while allowing air circulation through the solid particles. This practice would also be within the spirit of this disclosure. As in forced convection oven technology, the endeavor was to recycle the gas through the solid particles to achieve minimum discharge of gas from the system and thus avoid the use of energy to heat or cool gas expelled to the atmosphere or otherwise lost to the process and to avoid the power involved in passing much larger quantities of gas through pollution abatement equipment. In many cases, no gas flow through the container is needed; this is especially useful when heating or cooling with inert gases such as nitrogen or carbon dioxide. The present invention makes this possible because gas moving devices are positioned to blow or exhaust gas in a manner that said gas penetrates the cascading and falling solid particles and returns to the section of the container not filled with falling solid particles to be heated, cooled, humidified, or dehumidified, if needed, before being passed through the solid particles again.
One embodiment of this invention is now in operation, incorporated in a urea granulation pilot plant capable of three tons per hour, at the Tennessee Valley Authority in Muscle Shoals, Ala., and is being incorporated in the design of a 14 ton-per-hour plant under construction at the same location. The invention holds great promise for melt granulation and will no doubt take its place quickly as a major invention in that area; but the most dynamic aspect of this disclosure is the effect it will have on the total chemical process industry throughout the world wherever solid particles must be heated, dried, cooled, coated, or granulated.